1. Field of the Invention
This invention relates generally to waveguides and, more particularly, to tunable waveguide filters.
2. Description of the Related Art
Electromagnetic signals with wavelengths in the millimeter range are typically guided to a destination by a waveguide because of insertion loss considerations. An example of one such waveguide can be found in U.S. Pat. Nos. 6,603,357 and 6,628,242 which disclose waveguides with electromagnetic crystal (EMXT) surfaces. The EMXT surfaces allow for the transmission of high frequency signals with near uniform power density across the waveguide cross-section. More information on EMXT surfaces can be found in U.S. Pat. Nos. 6,262,495 and 6,483,480.
In some waveguide systems, filters are used to control the flow of signals during transmission and reception. The filters are chosen to provide low insertion loss in the selected bands and high power transmission with little or no distortion. A typical millimeter wave system includes separate waveguide and filter combinations, with each combination being sensitive to a different resonant frequency. The filters include a resonant cavity that can be tuned to a particular resonant frequency using mechanical adjustments such as tuning screws as disclosed in U.S. Pat. No. 5,691,677 or movable dielectric inserts as disclosed in U.S. Pat. Nos. 4,459,564 and 6,392,508. In both of these cases, tuning is accomplished by mechanically adjusting the screw or insert to change the length of the resonant cavity and the resonant frequency.
If the mechanical adjustment cannot tune the resonant frequency quickly enough, then more waveguide and filter combinations will be needed, with each one tuned for a different resonant frequency. For example, a single antenna can be coupled to separate filters and their corresponding waveguides. In this setup, one filter-waveguide combination can be tuned to transmit and receive communication signals in one frequency band and another can be tuned to transmit and receive radar signals in a different frequency band. It is desired, however, to reduce the number of waveguide-filter combinations needed to transmit signals over the different frequency bands.